The present invention is directed to a unique method and device for blood sample dilution, which can be used in an automatic hematology cell counter.
Before starting an analysis, a hematology cell counter must dilute the whole blood sample into two diluted samples, one of low dilution ratio for WBC (while blood cell)/HGB(hemoglobin) counting, one of high dilution ratio for RBC (red blood cell)/PLT (platelete) counting. From commonly used dilution methods, we may summarize two types of dilution methods, the Twice-Sampling method and the Single-Sampling method. Each method has its own advantages and drawbacks, describing as follows:
A. The Twice-Sampling method:
This method uses twice diluting to obtain high dilution ratio sample. The whole blood is aspirated through a sample probe by a sample syringe. After wiping off blood remaining on the outside the probe, the sampled blood inside the probe is dispensed by an isotonic diluent syringe to a bath. This diluted sample in the bath is the first low dilution ratio sample for WBC/HGB counting. Again, the probe aspirates the first diluted sample then sends it to the other bath. This second diluted sample is the high dilution ratio sample for RBC/PLT counting. Advantage: Low sample volume is needed, typically 20-40 .mu.l. Drawbacks: (a) Because of its complicated procedure, it has low efficiency and low throughput. (b) A cleaning or washing procedure must be carried out to clean the outside wall of the probe after sampling.(c) A premixing cup or sample bath is opened up to a large extent. Therefore, dust particles can clog the counting orifice.
B. The Single-Sampling method
The whole blood sample is directly dispensed into two sample volumes for two dilution ratios. Commonly, a blood sampling valve (B.S.V.) is used. A B.S.V. is made of ceramics, comprised of three perforated circular plates connected one above another. As the blood sample is delivered from the sample probe through the valve, the intermediate plate is turned to collect two separated sample volumes for two different dilution ratios, then the diluent is sent to respective holes on the valve to flush diluted samples into two counting baths respectively. The intermediate plate is then turned back to its blood sampling home position. And the blood sample passing path is cleaned by sending the diluent through the passage to the waste. Advantage: Only one sampling is used achieving higher efficiency and higher throughput. Drawbacks: (a) Because it has a long path for its blood sample sucking stroke, typically around 100 .mu.l, a large sample volume is needed. (b) Liquid leakage may occur during the rotary motion of the intermediate cylindrical section. Accordingly, the B.S.V. must be regularly maintained.